Conversion in chemical reactions is limited by the equilibrium position of the reaction. If the chemical equilibrium in a synthesis reaction is only partly to the side of the products, a one-stage reaction regime leads to partial conversion. Therefore, only a portion of the reactants is converted in one pass through the reactor.
Very many reactions are equilibrium-limited. In chemical synthesis, these are, for example, the production of methanol from hydrogen and carbon monoxide and/or carbon dioxide, or the production of ammonia from hydrogen and nitrogen. Nowadays, these reactions take place in heterogeneous catalyzed fixed bed reactors or slurry reactors. The reactants are only partly converted in a single pass through the reactor. Thereafter, the reaction products are typically removed and the unconverted reactants are recirculated to the reaction inlet.
The recirculation of volumes of gas that are large in some cases leads to high apparatus complexity. The pressure drop in the reactor is compensated for by a recirculation unit. This is usually operated at high temperatures and leads to high costs. Moreover, the recirculation results in accumulation of inert and extraneous gases in the circuit, which leads to adverse effects on the reaction regime. In the case of recirculation, therefore, a small amount of recirculation gas is drawn off continuously, which results in losses of reactants and hence lower reaction conversions. Moreover, the recycled gas volume leads to a high gas volume flow rate through the reactor, which increases the construction size and hence the costs of the reactor.